Magnetic Spring, A Spring  And Damper Assembly, And A Vehicle Including The Spring

ABSTRACT

Spring having a first part ( 1 ) and a second part ( 2 ) moveable relative to each other along a longitudinal axis (L), a gap ( 3 ) being provided between the first part ( 1 ) and the second part ( 2 ), the first part ( 1 ) being provided with permanent magnets ( 4,4′ ), the second part ( 2 ) including soft magnetic material, the second part ( 2 ) having a variable permeability along the longitudinal axis (L) thereof for a magnetic field provided by the permanent magnets ( 4,4′ ) of the first part ( 1 ), wherein the second part ( 2 ), for providing the variable permeability for magnetic field, is provided with at least two teeth ( 5,5′ ) of soft magnetic material which each face a permanent magnet ( 4,4′ ) with a facing surface when the first part ( 1 ) is in an aligned position relative to the second part ( 2 ), the gap extending between the two teeth and the corresponding permanent magnets, wherein, when the first part is moving out of the aligned position relative to the second part along a longitudinal axis, the total area of the facing surfaces varies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a spring, more particularly to a spring havinga stroke along a longitudinal axis and in which permanent magnets areused.

The invention also relates to an assembly of a spring and a damper andto a vehicle having a wheel and a wheel suspension in which a springaccording to the invention is integrated.

2. Description of the Prior Art

U.S. Pat. No. 5,584,364 describes a magnetic unit used as a shockabsorber or load leveler for a vehicle and having a plurality of groupsof permanent magnets arranged in a side-by side relationship.

EP-A-0 391 066 describes an integrated spring system comprising aconventional spring, a magnetic spring and a damper. The magnetic springhas an inner part and an outer part each provided with permanentmagnets, whereby the direction of the fields of the inner and outerparts are opposite so that a negative spring stiffness is provided,which, in combination with the conventional spring provides a springsystem with a non-linear spring characteristic. The spring stiffness issmall around the neutral or aligned position and is large when thespring is compressed further.

In both US'364 and EP'066 the repulsive force of two neighboringpermanent magnets, which are oppositely oriented, is used for evoking aspring effect.

SUMMARY OF THE INVENTION

An object of the invention is to provide a spring comprising acombination of hard and soft magnetic materials with which a largespring force response, more specifically increasing or decreasing forcewith displacement, can be provided while the volume of the spring isrelatively small.

To that end, the inventions provides a spring having a first part and asecond part moveable relative to each other along a longitudinal axis,at least one gap being provided between the first part and the secondpart, the first part being provided with permanent magnets of a hardmagnetic material, the second part including soft magnetic material, thesecond part having a variable permeability along the longitudinal axisthereof for a magnetic field provided by the permanent magnets of thefirst part, wherein the second part, for providing the variablepermeability for the magnetic field, is provided with at least two teethof soft magnetic material which each face a permanent magnet with afacing surface when the first part is in an aligned position relative tothe second part, the at least one gap extending between the at least twoteeth and the corresponding permanent magnets, wherein, when the firstpart is moving out of the aligned position relative to the second partalong a longitudinal axis, the total area of the facing surfaces varies.

Such a spring uses the so-called reluctance or detent force for evokingthe spring force, since the magnetic field produced by the permanentmagnets wants to maintain to extend through the path having the highestpermeability. When the second part is moved relative to the first part,the permanent magnets which are opposite the teeth are moved relative tothe teeth. Consequently, the total area of the surfaces of the permanentmagnets facing the surfaces of the teeth varies, hence the area throughwhich the magnetic field passes varies. The reluctance force or detenturges the first and the second part in a position wherein the area ofthe facing surfaces is maximized.

In view of the fact that the reluctance or detent force is a strongforce, a high spring force can be obtained in a relatively small volume.Another new effect is that more or less linear spring force behavior,i.e. constant spring stiffness, is obtained, which even occurs directlyaround the neutral or aligned position of the first and the second partrelative to each other. In normal mechanical springs, a linear behavioris only obtainable around a partly compressed, in case of a compressionspring, or partly extended in case of a draw spring. The springaccording to the invention provides a force in two directions, i.e. theforce is directed such that the first part is urged towards the neutralor aligned position relative to the second part. Conventional mechanicalsprings have a spring characteristic which extends in theforce/displacement-diagram in one quadrant, whereas the spring accordingto the invention has a spring characteristic which extends in theforce/displacement-diagram in two quadrants, i.e. the first and thethird quadrant.

In an embodiment of the invention the total area of the facing surfacesis maximal when the first part is in the aligned position relative tothe second part and diminishes when the first part is moving out of thealigned position relative to the second part along a longitudinal axis.

In such an embodiment, in the aligned position, the spring force iszero, at least when no other spring force generating devices, such asadditional conventional springs are present.

Preferably, the teeth of the second part are interconnected to eachother by a base of soft magnetic material. Further, preferably, thepermanent magnets are mounted on a base soft magnetic material. In thatmanner a closed path which is highly permeable to a magnetic field iscreated so that a closed magnetic field is obtained extending throughtwo neighboring teeth via the base of the second part, the permanentmagnets and the base of the first part. The reluctance or detent forcewhich is evoked in such a construction when the first part is movedrelative to the second part along the longitudinal axis is very highbecause the rate of change of permeability with position and because thedensity of the magnetic field is high.

Preferably, the dimension of a tooth in the longitudinal direction is A,the distance between two sides of two neighboring teeth which sides aredirected towards each other is B, and wherein the dimension of apermanent magnet in the longitudinal direction is C, wherein A≧(B or C).In such an embodiment, a permanent magnet corresponding to a first toothwill not interfere with a neighboring tooth, at least not when thespring stroke is kept within the boundaries set for the spring.

An optimal relation between spring volume and stroke length is obtainedin the last described embodiment when A=B and A≧C. With such a spring,the spring has a stroke in the direction of the longitudinal axis,wherein the length of the stroke is A. In the center of the stroke, thefirst part is in the aligned position relative to the second part andthe spring force is equal to zero. When the moveable one (the mover) ofthe first and the second part is moved out of the aligned position, thespring force is directed along the longitudinal axis so that the moveris urged to the aligned position.

In an embodiment of the invention the surface of a tooth facing a facingsurface of a corresponding permanent magnet is shaped according to acurve to obtain a desired spring force characteristic along thelongitudinal axis when the first part moves relative to the second partfrom the aligned position to a non-aligned position.

By shaping the teeth, the force/displacement-characteristic can beinfluenced. Embodiments of this principle are e.g.:

-   -   a spring wherein the teeth are block shaped so that the tooth        facing a facing surface of a corresponding permanent magnet is        straight and parallel to the longitudinal axis; or    -   the teeth are saw tooth shaped so that the tooth facing a facing        surface of a corresponding permanent magnet defines a triangular        tooth; or    -   the tooth facing a facing surface of a corresponding permanent        is defined by a circular segment, be it convex or concave        relative to the permanent magnet.

In an embodiment of the invention at least one of the first and secondparts is provided with a coil which is connectable to a electric sourcefor creating a magnetic field which counteracts the magnetic fieldcreated by the permanent magnets or which adds to the magnetic fieldcreated by the permanent magnets, therewith influencing the springforce/displacement-characteristic of the spring.

Generally, although not limited to, the electric source will be aDC-source. The great virtue of this embodiment is that springforce/displacement-characteristic can be influenced by varying thecurrent supplied to the DC-source. This variation can be done veryrapidly, i.e. with a very high frequency, and a very small responsetime. Consequently, when the spring is used in e.g. a vehiclesuspension, the spring force generated by the spring can, at any moment,be varied to adapt the desired spring force to the actual circumstancesof the moment. The spring in this embodiment can be given an activespring behavior.

From a constructional point of view it is preferable when the coil isintegrated in the stationary one, the stator, of the first and thesecond part.

In a further elaboration of the active spring embodiment, the source isconnected to a control for controlling the current delivered by thesource and, consequently, for controlling the springforce/displacement-characteristic of the spring. Preferably, the controlis adapted for varying the current of the source continuously, so thatthe spring force is continuously variable in any stroke position at anymoment. The spring force can be increased by controlling the electricsource to create a magnetic filed which adds to the magnetic fieldcreated by the permanent magnets and can be decreased by controlling theelectric source to create a magnetic field with the coil whichcounteracts the magnetic field created by the permanent magnets. Suchcurrent variation can be effected at any moment and at any relativeposition of the first part relative to the second part.

In order to provide a spring with a certain volume envelope which canexert a higher spring force, an embodiment of the invention provides aspring wherein multiple gaps are provided between the first and thesecond part, each gap being bounded by at least two teeth and twopermanent magnets each one corresponding to one of said two teeth.

A practical embodiment could be an inner cylinder and an outer hollowcylinder which are stationary and which carry the teeth and anintermediate hollow cylinder extending between the inner and the outercylinders, carrying the permanent magnets both on its inner and itsouter surface.

In order to provide a spring with a certain volume envelope which canexert a higher spring force, an embodiment of the invention provides aspring, wherein, viewed in the direction of the longitudinal axis in agap more than two teeth are provided on the second part and acorresponding number of permanent magnets is provided on the first part.

In order to provide a spring with a certain volume envelope which canexert a higher spring force, an embodiment of the invention provides aspring, wherein the second part is provided with hard magnetic materialat least between two neighboring teeth and downstream of the last toothseen in the stroke direction, the hard magnetic material having anorientation such that a repulsive force between that hard magneticmaterial and the permanent magnets of the first part is evoked when thefirst part is moved out of the aligned position relative to the secondpart.

In this embodiment the reluctance or detent force—which reluctance forceoccurs when the facing surfaces of the permanent magnets are moved awayfrom the facing surfaces of the teeth by the relative movement of thefirst part and the second part along the longitudinal axis—is combinedby the repulsive force which is evoked between the hard magneticmaterial on the second part and the permanent magnets of the first part.

Consequently, the spring force is both determined by the reluctanceforce and said repulsive force. This provides a spring with a higherspring characteristic within a certain volume envelop.

The invention also provides an assembly of a spring of one of thepreceding claims with a damping device. The damping device can be aseparate device from the spring or can be integrated in the spring.

The invention also provides a vehicle provided with at least one wheeland a suspension via which the wheel is connected to a chassis of thevehicle, the suspension comprising at least one spring according to theinvention. Especially when the spring is of the active type, i.e.comprises a coil which is controlled by a control for influencing thespring characteristic of the spring, an active suspension with a veryshort response time can be obtained while the spring is rather simpleand robust from a constructional point of view.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross section of a first embodiment of thespring in the aligned or neutral position;

FIG. 1A shows a similar view as in FIG. 1, wherein the first part ismoved out of the aligned position relative to the second part;

FIG. 2 shows a schematic cross section of a second embodiment of thespring;

FIG. 3 shows a force/displacement-diagram of an embodiment of thespring;

FIG. 4 shows a schematic cross section of a spring with three teeth;

FIG. 5 shows a schematic cross section of a spring with two gaps;

FIG. 6 shows in a perspective cross section a practical embodiment ofthe embodiment schematically shown in FIG. 1;

FIG. 7 shows in a perspective cross section a practical embodiment of aspring with two gaps;

FIG. 8 shows an embodiment in which the teeth have a saw tooth shape;

FIG. 9 shows an embodiment in which the teeth have circle segment curve;

FIG. 10 shows an embodiment in which the teeth taper in the direction ofthe longitudinal axis;

FIG. 11 shows a schematic cross section of an embodiment in whichpermanent magnets are included in the second part;

FIG. 12 schematically shows a vehicle with a wheel and a wheelsuspension; and

FIG. 13 schematically shows two wheels of a vehicle with an anti rollbar and an embodiment of the spring according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of FIG. 1 shows a spring having a first part 1 and asecond part 2 moveable relative to each other along a longitudinal axisL. In this embodiment, the first part 1 is a mover and the second part 2is a stator. The mover is guided in a guide which can be provided bybearings in a known manner. The bearing can be of various types such asball bearings, roller bearings, magnetic bearings, fluid bearings, slidebearings or the like. Between the first part 1 and the second part 2, agap 3 is present. The first part 1 is provided with permanent magnets 4,4′ of a hard magnetic material. In this embodiment, the permanentmagnets 4, 4′ are mounted on a base 6 of soft magnetic material. Thesecond part 2 includes and is preferably entirely made of soft magneticmaterial. The shape of the second part 2 is such that it has a variablepermeability along the longitudinal axis thereof for the magnetic field(indicated by the dotted line PMF) provided by the permanent magnets 4,4′ of the first part 1. For providing the variable permeability for themagnetic field, the second part 2 is provided with at least two teeth 5,5′ of soft magnetic material which each face a permanent magnet 4, 4′with a facing surface 5 a, 5 a′ when the first part 1 is in an alignedposition relative to the second part 2. The facing surfaces 5 a, 5 a′ ofeach tooth 5, 5′ are directed towards a facing surface or pole 4 a, 4 a′of the corresponding permanent magnets 4, 4′. Neighboring permanentmagnets 4, 4′ preferably have a opposite magnetic direction. In FIG. 1,the magnetic direction of each magnet 4, 4′ is indicated by an arrow inthe magnet 4, 4′. The north pole of the magnets 4, 4′ is indicated by Nand the south pole is indicated by S. The gap 3 extends between the twoteeth 5, 5′ and the corresponding permanent magnets 4, 4′. When thefirst part 1 is moving out of the aligned position (shown in FIG. 1)into a non-aligned position (shown in FIG. 1A) relative to the secondpart 2 along the longitudinal axis L, the total area of the facingsurfaces varies. In the embodiment shown in FIG. 1, the total area ofthe facing surfaces is maximal when the first part 1 is in the alignedposition relative to the second part 2 and diminishes when the firstpart 1 is moving out of the aligned position relative to the second part2 along the longitudinal axis L.

Due to reluctance or detent force, the first part 1 is urged towards thealigned position, thus providing a spring force in a direction parallelto the longitudinal axis L. For a certain volume envelope of the spring,the proposed solution using reluctance or detent force, provides arelatively high spring force characteristic. The spring has the specialfeature that it incorporates both a compression spring and a draw springcharacteristic. In other words, in a force/displacement-characteristic(see FIG. 3), the spring acts in the first and the third quadrant,whereby the linearity of the spring force/displacement-characteristicaround the aligned position is very good. The springforce/displacement-characteristic can be tailored to a desiredcharacteristic. This will be elucidated when referring to FIGS. 8-10.

The dimension of a tooth 5, 5′ in the longitudinal direction isindicated by A. The distance between two sides of two neighboring teeth5, 5′ which sides are directed towards each other is indicated by B. Thedimensions of the permanent magnets 4, 4′ in the longitudinal directionis C. In order to obtain a good spring characteristic it is preferredwhen A≧(B or C). In those circumstances interference between a permanentmagnet with a tooth which does not correspond to that permanent magnetis prevented.

It is even more preferred when A=B and A≧C. For such an embodiment, thespring has a stroke in the direction of the longitudinal axis, whereinthe length of the stroke is A.

When, for example, A>C, then movements around the aligned position willnot result in the evocation of reluctance forces and, consequently,spring forces. For some applications that can be desired, e.g. when acertain play around a neutral position is desired.

A practical embodiment of the embodiment of FIG. 1 is shown in FIG. 6 inwhich the second part 2 is a hollow, substantially cylindrical partwhich has the teeth 5, 5′ on its inner surface. The first part 1 is asubstantially cylindrical part which carries the permanent magnets 4, 4′on its outer surface. It should be noted that the principle embodimentschematically shown in FIG. 1 could also be created in a practicalembodiment when the first part 1 and the second part 2 are substantiallyflat and parallel to each other. Further non-cylindrical embodiments arefeasible, e.g. embodiments in which the first and second parts 1, 2 havea substantially rectangular or triangular cross section perpendicular tothe longitudinal axis. It should be noted that in stead of theembodiments shown in FIGS. 1-6, the first part carrying the permanentmagnets 4, 4′ can also be stationary while the second part carrying theteeth 5, 5′ is moveable. Such an embodiment is e.g. shown in FIG. 7.

FIG. 2 schematically shows a second embodiment in which a coil 7 isadded to the second part 1. The coil is connected to a variableelectrical source 8 which is controlled by a control 9. The coil 7, ofwhich only one part of the windings is shown in cross section in FIG. 2,produces a coil magnetic field CMF when a current is flowing through thecoil. The coil magnetic field can have the same direction as themagnetic field PMF produced by the permanent magnets 4, 4′. In thosecircumstances the spring force is increased relative to the passivecircumstances (no current through the windings of the coil). On theother hand, the coil magnetic field can have the opposite direction asthe magnetic field PMF produced by the permanent magnets 4, 4′. In thosecircumstances the spring force is diminished relative to the passivecircumstances. The direction of the coil magnetic field CMF isdetermined by the direction of the current through the windings of thecoil. The density of the coil magnetic field is determined by thecurrent level in the coil.

The control 9 is adapted to continuously control and vary when desiredthe current level and the current direction in the coil 7 and therewithcontrols the spring force of the spring.

Instead of a single characteristic p in the force/displacement-diagram(see FIG. 3) which would be obtained for a passive spring (i.e. notprovided with a coil), an entire spring force area in the first and thethird quadrant of the diagram is covered by a single spring. Theboundaries of the area are indicated by two lines act+ and act−. At anymoment, at any displacement position, the spring force exerted by thespring can be instantaneously adjusted by varying the current in thecoil 7. This can be effected at a very high rate with virtually noresponse time. Thus an active spring is obtained

From a constructional point of view it is preferable when the coil isintegrated in the stationary one, the stator 2, of the first and thesecond parts 1, 2 respectively. However, the invention also coversembodiments in which the coil is integrated in the moveable one, themover 1, of the first and the second parts.

In order to increase the spring force which can be exerted by thespring, embodiments as in FIG. 4 are feasible in which, viewed in thedirection of the longitudinal axis in a gap more than two teeth 5, 5′,5″ are provided on the second part 2 and a corresponding number ofpermanent magnets 4, 4′, 4″ is provided on the first part 1. Theembodiment of FIG. 4 shows 3 teeth 5, 5′, 5″. It will be clear thatlarger numbers of teeth are also possible.

Another way to increase the spring force which can be exerted by thespring is shown in FIGS. 5 and 7. In these embodiments, multiple gaps 3,3′ are present.

In the embodiment of FIG. 5, two stationary second parts 2, 2′ arepresent and one moveable first part 1 is present. The first gap 3 isbounded by two permanent magnets 4, 4′ and two teeth 5, 5′. The secondgap 3′ is bounded by two permanent magnets 4″, 4′″ and two teeth 5″,5′″. The spring force is doubled while the length of the construction isstill the same.

In the embodiment of FIG. 7, two stationary first parts 1, 1′ arepresent and one moveable second part 2 is present. An outer,substantially hollow cylindrical first part 1 carries permanent magnets4, 4′ on its inner surface. An inner, substantially cylindrical firstpart 1′ carries permanent magnets 4″, 4″′ on its outer surface. Anintermediate second part 2 has a substantially hollow cylindricalconfiguration and carries teeth 5, 5′, 5″, 5″′ both on its inner and itsouter surface.

The force/displacement-characteristic p, can be tailored as mentionedbefore. This can be done by shaping the curve of facing surface 5 a, 5a′ of the teeth 5, 5′. The embodiments of FIGS. 1-7 have block shapedteeth so that so that the curve of the facing surfaces 5 a, 5 a′ isstraight and parallel to the longitudinal axis L. In such an embodiment,the spring force characteristic is substantially linear, at least aroundthe aligned position. Further away from the aligned position, the springstiffness will decrease.

In the embodiment of FIG. 8, the teeth 5 a, 5 a′ are saw tooth shaped sothat the curve defines a triangular tooth. With such a saw tooth shape,the spring force stiffness will decrease quicker with a displacement outof the aligned position than with the block shaped teeth.

In the embodiment of FIG. 9, the curve of the facing surfaces 5 a, 5 a′of the teeth 5, 5′ is defined by a circular segment. The teeth 5, 5′ areconcave relative to the permanent magnets 4, 4′. In this embodiment, theforce/displacement-characteristic of the spring can be tailored. Whenthe circular segments define convex teeth 5, 5′ relative to thepermanent magnets 4, 4′, a behavior is obtained which is similar to thatof the embodiment of FIG. 8 with the saw tooth shaped teeth.

By tailoring the teeth a force/displacement-characteristic of apre-stressed spring can be mimicked, i.e. a small movement directlyaround the aligned position of the first part relative to the secondpart directly evokes a spring force. In the force/displacement-diagramthis results in a force/displacement-characteristic which does not crossthe cross point of the axes but which starts at the force axis above orbelow the displacement axis.

In the embodiment of FIG. 10, the teeth 5 a, 5 a′ and the permanentmagnets 4, 4′ taper relative to the longitudinal axis L. Consequently,when the first part 1 displaces downwardly relative to the second part 2in the longitudinal direction, the width of the gap 3 increases, whichcauses a tailored spring force/displacement-characteristic differentfrom the embodiments with the block shaped teeth in which the gap widthdoes not change when the mover moves relative to the stator. In theembodiment of FIG. 10 the spring stiffness will decrease quicker whenthe mover moves away from the aligned position than with embodimentshaving block shaped teeth.

Of course, it is alternatively or additionally possible to shape thecurve of facing surfaces 4 a, 4 a′ of the permanent magnets 4, 4′ totailor the spring force characteristics of the spring.

In order to increase the exerted spring force without increasing thespace volume envelop of the spring the embodiment of FIG. 11 isproposed. In that embodiment, the second part 2 is provided with hardmagnetic material 9, 9′ at least between two neighboring teeth 5, 5′ anddownstream of the last tooth 5′ seen in the stroke direction. Themagnetic direction of the hard magnetic material 9, 9′ is such that arepulsive force between that hard magnetic material 9, 9′ and thepermanent magnets 4, 4′ of the first part 1 is evoked when the firstpart 1 is moved out of the aligned position relative to the second part2. In this spring the spring force is a combination of the reluctanceforce—which reluctance force is evoked by the varying facing surfacearea of the teeth 5, 5′ and the permanent magnets 4, 4′—and therepulsive force between the hard magnetic material on the second part 2and the permanent magnets 4, 4′ on the first part 1. Due to thiscombination of forces a very high spring stiffness within a relativelysmall volume envelope is provided.

FIG. 12 schematically shows a vehicle V with a wheel W and a wheelsuspension T. The wheel suspension is provided with a spring S of thetype according to the invention. More particularly, the suspension isprovided with an assembly 11, comprising a spring S of the typeaccording to the invention and a damper 12. Although depictedseparately, the damper 12 can be integrated into the spring S. When thespring S is of the active type as described above, the springcharacteristic can be adapted to the driving circumstances of thevehicle V. For example roll can be counter acted in curves, diving ofthe vehicle during braking can be counter acted and the springforce/displacement characteristic can be adapted to the type of road,the speed and/or the way of driving, i.e. sportive or relaxed andcomfortable. All these variations can be effected instantaneouslywithout virtually any response time by means of the control 9 whichcontrols the current level and current direction in the coils 7 of thespring S.

FIG. 13 schematically shows an embodiment of a vehicle V provided with afirst and a second wheel W1, W2 and an anti roll bar 12. The vehiclecomprises anti roll bar spring 13 which is an embodiment of a springaccording to the invention and which is connected in series with theanti roll bar 12 between the first and the second wheels W1, W2.Additionally, a damper 14 and a conventional spring 15 or an additionalspring according to an embodiment of the invention can be provided forsuspension of the each wheel W1, W2. Especially when the anti roll barspring 13 is of the active type, the roll behavior of the vehicle V canbe adapted as desired, e.g. in dependence to the type of road, the speedand/or the way of driving, i.e. sportive or relaxed and comfortable. Allthese variations can be effected instantaneously without virtually anyresponse time

It is clear that the invention is not limited to the embodimentsdescribed above but is defined by the appended claims. Combinations ofthe various embodiments are also feasible. For example a multi gapspring having more than two teeth in each gap and having coils betweenthe neighboring teeth for creating a coil magnetic field is also withinthe scope of the present invention.

1. A spring comprising: a first part and a second part moveable relativeto each other along a longitudinal axis, at least one gap being definedbetween the first part and the second part, the first part includingpermanent magnets of a hard magnetic material, the second part includinga soft magnetic material, the second part having at least two teeth ofsoft magnetic material which each have a surface facing an adjacentsurface of one of the permanent magnets when the first part is in analigned position relative to the second part, the at least one gap beingdefined between the at least two teeth and the corresponding permanentmagnets, wherein, when the first part moves from the aligned positionrelative to the second part along the longitudinal axis, the total areaof the facing surfaces of the teeth and of the permanent magnets variessuch that the magnetic permeability of the second part in response to amagnetic field of the permanent magnets varies along the longitudinalaxis.
 2. The spring according to claim 1, wherein the total area of thefacing surfaces of the teeth and the permanent magnets is maximal whenthe first part is in the aligned position relative to the second partand diminishes when the first part is moves from the aligned positionrelative to the second part along a longitudinal axis.
 3. The springaccording to claim 1, wherein the teeth of the second part areinterconnected by soft magnetic material.
 4. The spring according toclaim 1, wherein the permanent magnets of the first part are mounted ona base of soft magnetic material.
 5. The spring according to claim 1,wherein the dimension of each tooth in the longitudinal direction is A,each tooth has at least one side facing generally toward another tooth,a distance between the sides of two neighboring teeth is B, and whereina dimension of each permanent magnet in the longitudinal direction is C,wherein A>(B or C).
 6. The spring according to claim 5, wherein A=B andA>C.
 7. The spring according to claim 6, wherein the spring has a strokein the direction of the longitudinal axis, and wherein the length of thestroke is A.
 8. The spring according to claim 1, wherein the facingsurface of each tooth is shaped according to a curve to obtain a desiredspring force characteristic along the longitudinal axis when the firstpart moves relative to the second part from the aligned position to anonaligned position.
 9. The spring according to claim 8, wherein eachtooth is block shaped so that the curve is straight and parallel to thelongitudinal axis.
 10. The spring according to claim 8, wherein the eachtooth is saw tooth shaped so that the curve defines a triangular tooth.11. The spring according to claim 8, wherein the curve is defined by acircular segment.
 12. The spring according to claim 1, wherein one ofthe first and second parts is provided with a coil which is connectableto an electric power source and configured to create a magnetic fieldwhich influences the spring force characteristic of the spring by one ofcounteracting the magnetic field created by the permanent magnets andadding to the magnetic field created by the permanent magnets.
 13. Thespring according to claim 12, wherein the coil is integrated in astationary one of the first part and the second part.
 14. The springaccording to claim 12, wherein the electric power source is connected toa control for controlling the current delivered by the source so as tocontrol the spring force characteristic of the spring.
 15. The springaccording to claim 14, wherein the control is adapted for varying thecurrent of the source continuously so that the spring force iscontinuously variable in any stroke position of the spring.
 16. Thespring according to claim 1 wherein multiple gaps are provided betweenthe first part and the second part, each gap being bounded by at leasttwo teeth and two permanent magnets, each one of the permanent magnetscorresponding to one of said two teeth.
 17. The spring according toclaim 1, wherein more than two teeth are provided on the second part,the teeth being spaced apart along the longitudinal axis, and acorresponding number of permanent magnets is provided on the first part,the more than two teeth and corresponding magnets defining a single gap.18. The spring according to claim 1, wherein the second part is providedwith hard magnetic material located at least between two neighboringteeth and located downstream of a last tooth defined in the strokedirection, the hard magnetic material of the second part beingconfigured such that the magnetic direction of the hard magneticmaterial is arranged to generate a repulsive force between that hardmagnetic material and the permanent magnets of the first part when thefirst part is moved out of the aligned position relative to the secondpart.
 19. The spring according to claim 1 in combination with a dampingdevice.
 20. A vehicle comprising: a chassis; at least one wheel; and asuspension connecting the wheel with the chassis and having at least onespring, including a first part and a second part moveable relative toeach other along a longitudinal axis, at least one gap being definedbetween the first part and the second part, the first part includingpermanent magnets of a hard magnetic material, the second part includinga soft magnetic material, the second part having at least two teeth ofsoft magnetic material which each have a surface facing an adjacentsurface of one of the permanent magnets when the first part is in analigned position relative to the second part, the at least one gap beingdefined between the at least two teeth and the corresponding permanentmagnets, wherein, when the first part moves from the aligned positionrelative to the second part along the longitudinal axis, the total areaof the facing surfaces of the teeth and of the permanent magnets variessuch that the magnetic permeability of the second part in response to amagnetic field of the permanent magnets varies along the longitudinalaxis.
 21. A vehicle comprising at least a first wheel and a secondwheel, an anti roll bar, and anti roll bar spring connected in serieswith the anti roll bar between the first wheel and the second wheel, theanti roll bar spring including a first part and a second part moveablerelative to each other along a longitudinal axis, at least one gap beingdefined between the first part and the second part, the first partincluding permanent magnets of a hard magnetic material, the second partincluding a soft magnetic material, the second part having at least twoteeth of soft magnetic material which each have a surface facing anadjacent surface of one of the permanent magnets when the first part isin an aligned position relative to the second part, the at least one gapbeing defined between the at least two teeth and the correspondingpermanent magnets, wherein, when the first part moves from the alignedposition relative to the second part along the longitudinal axis, thetotal area of the facing surfaces of the teeth and of the permanentmagnets varies such that the magnetic permeability of the second part inresponse to a magnetic field of the permanent magnets varies along thelongitudinal axis.